There are too many examples of young female (even some male) runners who run incredible times but then dramatically fall off and stop competing at that level. It's very possible she will never run sub 4. Mary Cain ran her PRs at age 17 and was done running at a high level a few years later. Alexa Efraimson ran her PRs around age 18/19 and was done about five years later after slowly declining. Addy is 19 right now, no guarantee she keeps getting faster. Huge upside, but it's still more likely she does not get significantly faster than it is that she'll break American Records at the senior level let alone a world record.